Clinical studies have demonstrated a significant loss of respiratory function with age that results in reduced quality of life, increased propensity for other diseases, and ineffective aerosol drug delivery for the treatment of obstructive respiratory diseases. However, the cellular and molecular basis for this age-dependent loss of respiration is unknown. Airway smooth muscle (ASM) contractile state plays a significant role in regulating respiration by influencing the airway diameter and hence ASM is the target of drugs used in the treatment of obstructive airway diseases. Herein we propose to establish molecular changes that occur in the ASM that account for loss of respiratory function in the elderly using airways and ASM obtained from rats and human subjects. Preliminary studies demonstrate diminished contractile and relaxation responses of ASM in aged (Old) rats compared to Young and Early Aged rats. Additional data suggest aging promotes phenotype switching in ASM in which the smooth muscle contractile phenotype is changed to a proliferative/synthetic phenotype. Expression of myosin heavy chain and smooth muscle a-actin (contractile phenotype marker proteins) is lower in ASM obtained from Old rats compared to Young rats. Furthermore, global transcriptome analyses of ASM cells reveal decreased expression of myostatin, a member of the TGF- family known to inhibit the proliferation of myocytes. Lastly, preliminary data from ASM cells suggest diminished intracellular signaling with age to both contractile and relaxant agents that activate G protein-coupled receptors (GPCRs) on ASM. Based on these studies we hypothesize that ASM undergoes phenotype modulation with age that results in decreased contractile and relaxant responsiveness. We further hypothesize that alterations in 3 key regulatory features of ASM contractile underlie this phenotype switch: 1) GPCR signaling/pharmaco-mechanical coupling; 2) electromechanical coupling; and 3) dynamic cytoskeleton assembly. Lastly, we hypothesize that reduced expression of ASM myostatin in ASM is the principal upstream mechanism driving phenotype modulation with age. In Specific Aim 1, we propose to establish changes in ASM phenotype using rat and human airways and ASM cells from three distinct age groups, using novel tools such as myograph, optical magnetic twisting cytometry and traction microscopy. In Specific Aim 2, we propose to establish the mechanistic basis by which age causes a loss of the contractile/relaxant capacity of ASM by identifying age-related changes in GPCR signaling and pharmaco-mechanical coupling, regulation of membrane de/hyper-polarization and its effectors, and in the regulation of ASM cell cytoskeleton assembly. Aim 3 will explore the mechanistic role of myostatin as the principal upstream driver of the age-related change in ASM phenotype and those mechanisms underlying contractile function detailed in Aim 2. Collectively, these studies seek to identify age-dependent molecular changes in ASM function that contribute to the age-associated decline in respiratory function in the elderly population. Our findings may help develop tools to improve respiratory functions, and modify diagnostic and treatment regimens for obstructive pulmonary diseases in the elderly.